I. Field of the Invention
The present invention relates to a method for manufacturing gallium phosphide single crystals and, more particularly, to a method for manufacturing gallium phosphide single crystals by the pulling process.
II. Description of the Prior Art
Gallium phosphide (GaP) single crystals are widely used for manufacturing light-emitting diodes (LEDs). The GaP single crystals are conventionally manufactured by the rotary pulling process from liquid GaP. Since the melting point of GaP is as high as 1,467.degree. C. and the decomposition pressure of GaP at this melting point is as high as about 32 atmospheres, there is conventionally adopted a method for encapsulating the GaP liquid by an encapsulating agent (liquid encapsulated Czochralski method). According to this method, the polycrystalline GaP raw material which is prepared in advance and the encapsulating agent are charged in a crucible which is arranged in a pressure-resistant vessel. The pressure-resistant vessel is pressurized by an inert gas. When the content in the crucible is heated under this condition, the encapsulating agent which has the lower melting point melts and covers the polycrystalline GaP raw material. The crucible is further heated to melt the polycrystalline GaP raw material and to provide a GaP liquid. During this procedure, the encapsulating agent liquid and the GaP liquid form separate layers due to the difference in specific gravity. The encapsulating agent liquid layer covers the GaP liquid layer and prevents the decomposition of the GaP liquid. Thereafter, through the encapsulating agent liquid layer, a seed crystal is brought into contact with the GaP liquid and is slowly pulled while being rotated. In this manner, the single crystal is grown while the GaP liquid solidifies.
As for the method for manufacturing the polycrystalline gallium phosphide raw material, a direct method is known wherein gallium and phosphorus are directly reacted in a pressure-resistant vessel. However, since this reaction is performed under high pressure and high temperature, various drawbacks are involved. The reaction apparatus becomes expensive to manufacture and complex in structure. Therefore, the manufacturing cost of this raw material becomes high. Even if gallium and phophorus of high purity such as above 99.9999% are used for manufacturing the raw material, the gallium phosphide obtained from the reaction apparatus necessarily has a low purity of less than 99.999% due to unavoidable contamination.
In order to solve this problem, a method for manufacturing polycrystalline gallium phophide by hydrogen reduction has been proposed (Japanese Patent Publication (KOKOKU) No. 13,880/79) according to which gallium phosphate is manufactured in advance and hygrogen gas is supplied to the gallium phosphate while keeping the gallium phosphate at a relatively low temperature of 1,050.degree. C. for reaction. Efforts are being made at utilizing inexpensive polycrystalline GaP powder manufactured in this manner. However, the bulk density of the GaP powder thus obtained is as low as about 1.1 g/cc which is about 1/4 the theoretical density. When the GaP powder is placed in the crucible and melted, the gas entrapped in the GaP powder forms bubbles which ascend through the encapsulating agent liquid layer. When this happens, most of GaP becomes attached to the wall surface portion of the crucible which is above the encapsulating agent liquid layer. In order to solve this problem, the GaP may be formed into a mass of high density. However, when the GaP powder is mechanically compressed, the bulk density rises to 2.0 to 3.0 g/cc at most, and this is not satisfactory either. Japanese Patent Publication (KOKAI) Nos. 129,174/75 and 129,175/75 propose a method for preparing a mass of high density of GaP powder using B.sub. 2 O.sub.3 as a binder. However, according to this method, when the mass is melted, B.sub.2 O.sub.3 and GaP are not separated well, and the GaP powder is mixed in the B.sub.2 O.sub.3 liquid layer. The GaP powder mixxed in the B.sub.2 O.sub.3 decomposes when it is subjected to a high temperature during the melting procedure, contaminating the B.sub.2 O.sub.3 liquid and the GaP liquid. As a consequence, monitoring of the single crystal growth interface becomes difficult. In addition to this, the contamination becomes the source of formation of polycrystalline nuclei, extremely degrading the manufacturing yield of the single crystals.